1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method, and more particularly relates to a technology that reduces granularity in a printed image by performing multi-pass printing in which scanning by a print head is performed over the same area of a print medium multiple times to complete printing.
2. Description of the Related Art
In multi-pass printing, since the conveying of a print medium is performed between a plurality of scans to complete printing, ink droplets are applied to the print medium, in each scan, following the elapse of a comparatively long time period that includes time for conveying the print medium. Thus, there is a comparatively extended interval, between the ejection of ink for one scan and the ejection of ink for the next scan, that includes the time required to convey the print medium, and the absorption of ink droplets applied to the print medium can be accelerated.
The multi-pass printing system is useful for the absorption of ink, and rarely causes beading, which is a phenomenon where ink dots, which have not yet been fully absorbed, are coupled together. When the number of scans (hereinafter, also referred to as the number of passes) for completing printing is increased, the amount of ink to be applied during each scan is reduced, and the beading of ink occurs less often.
For example, in Japanese Patent Laid-Open No. 2000-135782, an ink jet printing method is disclosed, according to which different types of print medium having different ink absorption properties are employed for printing, and the number of passes is increased for a print medium having a lower absorption property to reduce the occurrence of beading.
However, inventors of the present invention studied this method and found another granularity problem that deteriorates the quality of an image provided by multi-pass printing. It is known that this new granularity problem is not appropriately resolved merely by increasing the number of passes in multi-pass printing. This problem is especially noticeable in a case wherein ink containing a pigment as a color material is employed for glossy paper.
FIG. 1 is a diagram for explaining a mechanism concerning how this granularity occurs. FIG. 1 shows that an ink droplet previously landed on a print medium 101 to form a colored portion 102, and another ink droplet 103 has landed on a portion of the colored portion 102. In the colored portion 102, as an ink solvent is permeating and is evaporating, a pigment that is an ink color material and solid portions such as soluble polymer having a function for dispersing the pigment, are coagulated and deposited. Since the permeation by ink of a print medium is greatly affected by the capillarity of the pore structure of the print medium, the permeating rate of ink at the colored portion 102 where the print medium is not exposed is sharply reduced as fixing time elapses. Whereas the permeating rate at a non-colored portion 104 is greater than that at the colored portion 102, and therefore, as time elapses, the ink droplet 103, which landed later, more permeates the uncolored portion 104 than the colored portion 102. As a result, deposition of the color material of ink landed later is hard to occur in the colored portion where ink landed first, and the size of an area of fixing ink that landed later is reduced.
Assume that the lightness of the color of ink that lands first is higher than the lightness of the color of the ink that lands later. In this case, because of the above described mechanism, ink that landed later is rarely deposited on a colored portion of an ink having a higher lightness, and thus the color of ink that landed later does not sufficiently appear. As a result, the color of the ink having the higher lightness, which landed first, is preferentially exhibited, and such an area is represented as being whitish and is viewed as a whitish granularity (hereinafter, also referred to as “apparent white patches”) throughout the entire printed image, so that image quality is deteriorated.